In a networking environment of fiber to the distribution point (FTTdp), a network device is usually disposed far away from a power source, such as outdoors, in a passageway, or between floors, so it is difficult for the network device to be powered.
A power over Ethernet (POE) technology allows a power sourcing equipment (PSE) to transmit data and at the same time directly supply, through the same Ethernet cable, power to a network device which has a power interface and may be used as a powered device (PD), thereby enabling the network device to take power through a lower-level device (which is usually disposed inside a room of a user, can easily connect to a power source, and is connected to the network device through an Ethernet cable) of the network device.
At present, there are two types of POE standards: 802.3af and 802.3at. A difference between the standard 802.3at and the standard 802.3af lies in that a highest grade of power in the standard 802.3at may reach 25.5 Watts (W), while a highest grade of power in the standard 802.3af only reaches 12.95 W.
Because the power sourcing equipment and the powered device are developed independently, using the research and development of the powered device as an example, the developer of the powered device cannot foresee the standard on which the power sourcing device used by a customer is based; and if the customer connects a powered device designed on the basis of the standard 802.3at to a power sourcing equipment designed on the basis of the standard 802.3af, it is possible that overload power-off is caused, because the powered device requests a power of 25.5 W, but the power sourcing equipment cannot provide a power exceeding 12.95 W.